Coloured diamond

ABSTRACT

A diamond layer of single crystal CVD diamond which is coloured, preferably which has a fancy colour, and which has a thickness of greater than 1 mm.

BACKGROUND OF THE INVENTION

This invention relates to a method of producing by chemical vapourdeposition (hereinafter referred to as CVD) coloured single crystaldiamond, and in one aspect a method of producing fancy coloured diamondsthese coloured diamonds being suitable, for example, for preparation forornamental purposes or applications in which colour is a secondaryparameter that may influence market acceptance.

Intrinsic diamond has an indirect band gap of 5.5 eV and is transparentin the visible part of the spectrum. Introducing defects or colourcentres, as they will be called, which have associated energy levelswithin the band gap gives the diamond a characteristic colour which isdependent on the type and concentration of the colour centres. Thiscolour can result from either absorption of photoluminescence or somecombination of these two. One example of a common colour centre presentin synthetic diamond is nitrogen which, when sitting on a substitutionallattice site in the neutral charge state, has an associated energy level˜1.7 eV below the conduction band—the resulting absorption gives thediamond a characteristic yellow/brown colour.

Methods of depositing material such as diamond on a substrate by CVD arenow well established and have been described extensively in the patentand other literature. Where diamond is being deposited on a substrate,the method generally involves providing a gas mixture which, ondissociation, can provide hydrogen or a halogen (e.g. F,CI) in atomicform and C or carbon-containing radicals and other reactive species,e.g. CH_(x), CF_(x) wherein x can be 1 to 4. In addition, oxygencontaining sources may be present, as may sources for nitrogen, and forboron. Nitrogen can be introduced in the synthesis plasma in many forms;typically these are N₂, NH₃, air and N₂H₄. In many processes inert gasessuch as helium, neon or argon are also present. Thus, a typical sourcegas mixture will contain hydrocarbons C_(x)H_(y) wherein x and y caneach be 1 to 10 or halocarbons C_(x)H_(y)Hal_(z) wherein x and z an eachbe 1 to 10 and y can be 0 to 10 and optionally one or more of thefollowing; CO_(x), wherein x can be 0,5 to 2, O₂, H₂, N₂, NH₃, B₂H₆ andan inert gas. Each gas may be present in its natural isotopic ratio, orthe relative isotopic ratios may be artificially controlled; for examplehydrogen may be present as deuterium or tritium, and carbon may bepresent as ¹²C or ¹³C. Dissociation of the source gas mixture is broughtabout by an energy source such as microwaves, RF (radio frequency)energy, a flame, a hot filament or jet based technique and the reactivegas species so produced are allowed to deposit onto a substrate and formdiamond.

CVD diamond may be produced on a variety of substrates. Depending on thenature of the substrate and details of the process chemistry,polycrystalline or single crystal CVD diamond may be produced.

It is well known that post growth treatment such as irradiation withsufficiently energetic particles (electron, neutron etc) to producelattice defects (interstitials and vacancies) and suitable annealing canresult in the formation of colour centres, such as the nitrogen vacancy[N-V] colour centre, which can give the diamond a desirable colour (seefor example EP 0 615 954 A1, EP 0 326 856 A1 and the references citedtherein). Further characteristics and artificial production of colourcentres are discussed in detail by John Walker in the Reports onProgress in Physics, Vol. 42 1979. The artificial production method ofcolour centres outlined therein comprises the steps of forming latticedefects in crystals by electron beam irradiation and, if necessaryannealing to cause the lattice defects to combine with nitrogen atomscontained in the crystals. However, there are limitations to the coloursand uniformity that can be produced as a consequence of competitivedefect formation and because of the strong sector dependence associatedwith defects such as nitrogen in diamond.

The colour of a diamond produced by utilising this post growth colourcentre formation method is the colour of the rough diamond combined withthe colour of the colour centre produced. In order to obtain theornamental value desired, and thus achieve a combination of hightransparency and fancy colour, it has been usual practice to usediamonds that were initially either transparent or light yellow.

There are three visual attributes to colour: hue, lightness andsaturation. Hue is the attribute of colour that allows it to beclassified as red, green, blue, yellow, black or white, or a hue that isintermediate between adjacent pairs or triplets of these basic hues(Stephen C. Hofer. Collecting and Classifying Coloured Diamonds, 1998,Ashland Press, New York).

White, grey and black objects are differentiated on a lightness scale oflight to dark. Lightness is the attribute of colour that is defined bythe degree of similarity with a neutral achromatic scale starting withwhite and progressing through darker levels of grey and ending withblack.

Saturation is the attribute of colour that is defined by the degree ofdifference from an achromatic colour of the same lightness. It is also adescriptive term corresponding to the strength of a colour. The diamondtrade uses adjectives such as intense, strong and vivid to denotedifferent degrees of saturation assessed visually. In the CIELAB coloursystem, saturation is the degree of departure from the neutral colouraxis (defined by saturation=[(a*)²+(b*)²]^(1/2), see hereinafter).Lightness is a visual quality perceived separately from saturation.

The dominant colour of much of the diamond of the invention describedhereinafter is brown. Brown is generally a darker, less saturatedversion of orange. As brown becomes lighter and more saturated itbecomes orange. Brown colours also underlie a portion of the yellow huefamily so that orange-yellow and orangish yellows in their darker andweaker variants may fall into the brown region.

For diamonds, intermediate colour descriptions between brown and orangeare used. In order of decreasing browness and increasing orangeness, thedescription of the colour goes through the following sequence: brown,orangish brown, orange-brown, brown-orange, brownish orange, orange.Similar sequences apply for the transitions from brown to orange-yellowor orangish yellow. In three-dimensional colour space the region ofbrown colours is also bordered by pink colour regions and on moving frombrown to pink the following sequence is followed: brown, pinkish brown,pink-brown, brown-pink, brownish pink, pink.

Fancy coloured diamonds are diamonds with an obvious and unusual colour.When the dominant component of that colour is brown they are describedas fancy brown. This term covers a complex range of colours, defined bya three dimensional region of colour space. It covers large ranges inthe values of lightness, hue and saturation.

The inherent colour of a cut diamond, sometimes called the body colour,can best be judged if the diamond is viewed from the side for typicalcuts. The apparent colour seen in the face-up direction (ie lookingtowards the table) can be greatly affected by the cut of the stonebecause of the effect that this has on the path length within the stonefor the light subsequently reaching the eye. For example, inherentlyorange-brown diamond can be cut in such a way that its face-up colourappears brighter, resulting in a reversal of the dominant colour tobrown-orange.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided acoloured single crystal diamond layer, preferably a fancy colouredsingle crystal diamond layer, and more preferably a fancy colouredsingle crystal diamond layer where brown is the dominant colour,synthesised by CVD and prepared or suitable for preparation as a cutstone for ornamental application or for other applications where colourmay influence market acceptance. The CVD diamond layer of the inventionpreferably has a hue angle of less than 80°, preferably a hue angle ofless than 75° and more preferably a hue angle of less than 70°. The hueangle for a particular hue can be found by extending the line back fromthe point representing that hue on the a* b* colour plot as describedmore fully hereinafter, and shown on FIG. 3.

The CVD diamond layer of the invention has a thickness greater than 1mm, and preferably greater than 2 mm and more preferably greater than 3mm.

The CVD diamond layer of the invention may also have one or more of thefollowing characteristics (i) (ii) and (iii) observable in the majorityvolume of the layer, where the majority volume comprises at least 55%,and preferably at least 80%, and more preferably at least 95% of thewhole volume of the layer. Preferably the majority volume of the layeris formed from a single growth sector.

(i) The majority volume of the CVD diamond layer contains one or more ofthe defect and impurity related colour centres that contribute to theabsorption spectrum of the diamond as set out in the absorptioncoefficient column of the table below: Designation Starts Ends PeakAbsorption coefficient (at peak) 270 nm 220 nm 325 nm 270 nm 0.1 cm⁻¹-30cm⁻¹ preferably 0.4 cm⁻¹-10 cm⁻¹ more preferably 0.8 cm⁻¹-6 cm⁻¹ 350 nmband 270 nm 450 nm 350 nm +/− 10 nm 0.3 cm⁻¹-20 cm⁻¹ preferably 1.0cm⁻¹-8 cm⁻¹ more preferably 1.5 cm⁻¹-6 cm⁻¹ 510 nm band 420 nm 640 nm510 nm +/− 50 nm 0.1 cm⁻¹-10 cm⁻¹ preferably 0.2 cm⁻¹-4 cm⁻¹ morepreferably 0.4 cm⁻¹-2 cm⁻¹ 570/637 nm 500 nm 640 nm 570 nm 0.1 cm⁻¹-5cm⁻¹ preferably 0.3 cm⁻¹-3 cm⁻¹ more preferably 0.3 cm⁻¹-1.5 cm⁻¹Designation Form of Curve Absorption Coefficient Ramp Rising backgroundof form Contribution at 510 nm is: Absorption coefficient (cm⁻¹) = C ×λ⁻³   <3 cm⁻¹ (C = constant, λin μm) preferably <1.5 cm⁻¹ morepreferably <0.8 cm⁻¹

(ii) The majority volume of the CVD diamond layer contains defect andimpurity related centres that contribute to the luminescence spectrum asdetailed in the normalised luminenscence intensity column of the tablebelow, when measured in the prescribed manner using Ar ion 514 nm laserexcitation at 77K and normalised relative to the Raman scatteringintensity: Normalised luminescence intensity Designation Starts EndsPeak of zero phonon line at 77K 575 nm 570 nm 680 nm 575 nm 0.02-80 preferably 0.05-60  more preferably 0.2-40 637 nm 635 nm 800 nm 637 nm0.01-300 preferably 0.02-200 more preferably 0.03-100

-   -   (iii) The majority volume of the CVD diamond layer exhibits a        ratio of normalised 637 nm/575 nm luminescence, measured in the        manner described herein, which is in the range 0.2-10, and        preferably in the range 0.5-8, and more preferably in the range        2-5.

The present invention provides, according to another aspect, colouredsingle crystal CVD diamond which has a low ramp value as defined abovein combination with a defect and impurity related colour centre thatcontributes to the absorption spectrum of the diamond at one or more of270 nm, 350 nm band, 510 nm band and 570/637 nm, as set out in the tableforming part of characteristic (i). The low ramp value in combinationwith one or more of the absorption spectrum characteristics provides thediamond with a desirable colour. The diamond will generally be in layerform. The thickness of the layer may range from a few microns to severalmm in thickness.

The present invention provides a coloured single crystal CVD diamondwhich is desirable. A particular aspect of the invention is theprovision of fancy coloured diamond suitable to produce gemstones, theterm fancy referring to a gem trade classification of stronger and moreunusual colours in diamond. Even more particularly the invention canprovide a range of fancy brown colours, an example being fancy lightpink brown. This invention further provides for a thick (>1 mm) diamondlayer with uniform properties through its thickness so that anydesirable colour is not quenched or hidden by defects related to lowcrystalline quality. The fanciness of the colours was originally notanticipated, nor the degree to which they could be controlled bychoosing appropriate synthesis and substrate conditions. No post growthtreatment is needed to produce these colours. In fact many of thesecolours are impossible to produce using post growth treatments, as aconsequence of the relative colour centre formation mechanisms thatcompete during irradiation and annealing. In addition characteristicsassociated with the CVD growth mechanism can result in absorption bandsat ˜350 nm and ˜510 nm. These are important for the final colourproduced, but the centres responsible are not present in natural orother synthetic diamond. Consequently the colours achieved are unique toCVD diamond, and more particularly to CVD diamond of the invention.

Further, there is no post growth treatment as the colour centres areintroduced by a careful selection of growth conditions. There are manyreports in the literature of homoepitaxial CVD growth on high pressurehigh temperature (HPHT) synthetic and natural diamond substrates.Although there are only a few reports of thick layers (>100 μm), thesetend to have an unattractive brown colour which results mainly fromabsorption related to low crystalline quality defects andgraphitic/metallic inclusions and which tend to increase with growththickness. Even if growth conditions were chosen to allow incorporationof colour centres that would give the diamond a desirable colour, thisdesirable colour would be masked by the dominant absorption relating tothe low quality nature of the diamond crystal structure.

In addition, the majority volume of the CVD diamond layer of theinvention may exhibit one or more of the following properties:

-   -   1. High crystalline quality as determined by a low density of        extended defects, related factors such as narrow Raman line        width, relatively featureless X-ray topography and narrow        rocking curve, mechanical integrity, strength and mechanical        processability of the material to form highly polished surfaces        and edges. In this context high quality excludes quality factors        normally requiring the absence of N, including features such as:        the N impurities themselves and also associated point defects        including H related defects and vacancies, electronic based        properties such as mobility and charge collection distance which        are very sensitive to scattering centres and traps, and the        specific optical absorption and luminescence characteristics        induced by the presence of the added nitrogen and the associated        defects.    -   2. A level of any single impurity: Fe, Si, P, S, Ni, Co, Al, Mn        of not greater than 1 ppm and a total impurity content of not        greater than 5 ppm. In the above, “impurity” excludes hydrogen        and its isotopic forms.    -   3. In EPR, a spin density <1×10¹⁷ cm⁻³ and more typically        <5×10¹⁶ cm⁻³ at g=2,0028. In single crystal diamond this line at        g=2.0028 is related to lattice defect concentrations and is        typically large in natural type IIa diamond, in CVD diamond        plastically deformed through indentation, and in poor quality        homoepitaxial diamond.    -   4. X-ray topography showing features related to growth where        <100> edges of the original substrate are grown out to form        <110> edges.

The coloured CVD diamond layer of the invention may be on a surface of asubstrate, typically a diamond substrate, and will generally be a freestanding layer. A gemstone can be produced from the composite CVDdiamond layer/diamond substrate or from the free standing layer.

The coloured single crystal CVD diamond of the invention may be made bya method that forms yet another aspect of the invention. This methodincludes the steps of providing a diamond substrate having a surfacewhich is substantially free of crystal defects, providing a source gas,dissociating the source gas to produce a synthesis atmosphere whichcontains 0,5 to 500 ppm nitrogen, calculated as molecular nitrogen, andallowing homoepitaxial diamond growth on the surface which issubstantially free of crystal defects.

In the method of the invention, the source gas which is used to producethe synthesis atmosphere in which homoepitaxial growth on the diamondsubstrate occurs contains a suitable amount of nitrogen. The nitrogenmay be included in the source gas or added to a source gas whichcontains substantially no nitrogen. The nitrogen, either in the sourcegas or added to the source gas, must be such as to produce a synthesisatmosphere which contains 0,5 to 500 ppm, preferably 1 to 100 ppm ofnitrogen, calculated as molecular nitrogen. The nitrogen in the sourcegas or added to it may be molecular nitrogen or a nitrogen containinggas such as ammonia.

The nitrogen in the synthesis atmosphere or plasma, in addition toproducing colour centres in the diamond, can be used beneficially tocause morphological changes to the growing single crystal CVD diamond.Specifically, the addition of nitrogen to the gas phase can be used toenhance the size of the {100} growth sector and reduce the size ofcompeting growth sectors such as the {111}. This means that, for growthon a {100} plate, the addition of nitrogen enables the growth to remainsubstantially {100} growth sector.

Coloured gemstones and more particularly fancy coloured gemstones may beproduced from the CVD diamond of the invention and CVD diamond producedby the method described above. Such gemstones may be of high quality. Ingem quality grading, one of the four key quality parameters is theclarity of the diamond gemstone. The clarity grades used are generallythose defined by the GIA (Gemological Institute of America) and run on ascale from FL (flawless), IF, VVS1 (very very slightly included), VVS2,VS1 (very slightly included), VS2, SI1 (slightly included), SI2, I1(imperrect), I2 and I3.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Spectral decomposition of UV/visible absorption spectrum of anorangish brown CVD diamond layer Spectrum A: Type Ib HPHT syntheticdiamond Spectrum B: Original spectrum of orangish brown CVD diamondSpectrum C: Spectral component with (wavelength)⁻³ dependence SpectrumD: Spectral component composed of two broad absorption bands

FIG. 2 UV/visible absorption spectra for a set of brown CVD layers

FIG. 3 CIELAB a*b* diagram for brown CVD diamond

FIG. 4 CIELB L*C* diagram for brown CVD diamond

FIG. 5 CIELAB a*b* plot for diamond layers grown in different kinds ofCVD process

FIG. 6 CIELAB L*C* plot for diamond layers grown in different kinds ofCVD process

FIG. 7 Spectral decomposition of UV/visible spectrum of FN-1 Spectrum A:FN-1 Spectrum B: Type Ib HPHT synthetic diamond Spectrum C: Spectralcomponent with (wavelength)⁻³ dependence Spectrum D: Spectral componentcomposed of two broad absorption bands

FIG. 8 CIELAB a*b* diagram from FN-1

FIG. 9 CIELAB lightness saturation diagram for FN-1

DETAILED DESCRIPTION OF THE INVENTION

Absorption Spectroscopy Of Homoepitaxial CVD Diamond

The UV/visible absorption spectrum of type Ib diamond contains featuresassociated with single substitutional nitrogen These include anabsorption coefficient maximum at 270 nm and, to longer wavelengths, agradual decrease in absorption coefficient between approximately 300 nmand 500 nm, with signs of a broad absorption band at approximately 365nm. These features can be seen in absorption spectra of a type Ib highpressure high temperature diamond such as spectrum A in FIG. 1. Althoughthe effect of single substitutional nitrogen on the absorption spectrumis greatest in the ultra-violet, it is the weaker absorption thatextends into the visible region of the spectrum that affects the colourof the type Ib diamond and gives it a characteristic yellow/browncolour. This particular colour when strong and displaying the brownelement is generally judged to be undesirable in a gemstone.

The UV/visible absorption spectrum of homoepitaxial CVD diamond dopedwith nitrogen typically contains a contribution from singlesubstitutional nitrogen with the spectral characteristics describedabove, In addition to single substitutional nitrogen, nitrogen dopedhomoepitaxial CVD diamond typically contains some nitrogen in the formof nitrogen vacancy centres. When the N-V centre is electrically neutral[N-V]⁰ it gives rise to absorption with a zero phonon line at 575 nm.When the N-V centre is negatively charged [N-V]⁻it gives rise toabsorption with a zero-phonon line at 637 nm and an associated system ofphonon bands with an absorption maximum at approximately 570 nm. At roomtemperature, the normal temperature for observation of gemstones, theabsorption bands of these two charge states of the N-V centre merge intoa broad band from about 500 nm-640 nm. This absorption band is in theyellow part of the visible spectrum, and when it is strong the crystalscan exhibit a complementary pink/purple colour. This absorption can playan important part in determining the colour of the diamond of thisinvention.

The UV/visible absorption spectra of low quality homoepitaxial CVDdiamond, show a gradual rise in measured absorption from the red to theblue region of the spectrum and into the ultra-violet. There may also becontributions from scattering. The spectra generally contain no otherfeatures, apart from those related to single substitutional nitrogen.This absorption spectrum gives an undesirable brown colour and suchdiamond often contains clearly visible graphitic inclusions. Suchdiamond is unsuitable as a gemstone material for these reasons andbecause it cannot in general be grown to substantial thicknesses withoutsevere degradation of the crystal quality.

The coloured single crystal CVD diamond of the invention is of highcrystalline quality and is substantially free of extended crystaldefects and defects that tend to degrade the colour. The absorptionspectrum of the nitrogen-doped diamond of the current invention containsadditional contributions that are not present in natural, HPHT syntheticdiamond or low quality CVD diamond. These include two broad bandscentred at approximately 350 nm and 510 nm. The band at approximately350 nm is distinct from the broad feature in that region of the spectrumof ordinary type Ib spectrum and distorts the spectrum of ordinary typeIb diamond to an extent dependent on the concentration of the centreresponsible relative to the single substitutional nitrogen.

Similarly the band centred at approximately 510 nm can overlapabsorption relating to negative nitrogen-vacancy centres and the visibleabsorption relating to single substitutional nitrogen.

The overlapping of the various contributions to the absorption spectracan cause the bands at approximately 350 and 510 nm to give rise tobroad shoulders in the absorption spectrum rather than distinct maxima.These contributions to absorption do however have a very significanteffect on the relative absorption coefficients of the diamond atwavelengths in the spectral region between 400 and 600 nm where the eyeis very sensitive to small differences. They therefore make an importantcontribution to the perceived colour of the diamond. Together with theluminescence characteristics noted below, these absorptioncharacteristics can give diamond gemstones produced from such diamonddesirable fancy brown colours, including fancy dark browns orange brownand pink brown.

The width and position in the spectrum of these bands can vary. Theposition of peak maxima is most easily ascertained by using the seconddifferential of the spectrum. It has been found that absorption spectracan generally be deconstructed into the following approximatecomponents.

-   1) Single substitutional nitrogen component with an absorption    coeffident at 270 nm that is generally within the range 0.4 cm⁻¹ and    10 cm⁻¹ and an absorption coefficient at 425 nm that generally lies    between 0.04 cm⁻¹ and 1 cm⁻¹.-   2) An absorption band centred at 3.54 eV (350 nm)+/−0.2 eV with a    FWHM of approximately 1 eV and a maximum contribution to the    absorption spectrum generally between 1 and 8 cm⁻¹ at its centre.-   3) absorption band centred at 2.43 eV (510 nm)+/−0.4 eV with a FWHM    of approximately 1 eV and a maximum contribution to the absorption    spectrum generally between 0.2 and 4 cm⁻¹ at its centre.-   4) A small residual wavelength dependent component of the measured    absorption coefficient (in cm⁻¹) that is found to have a wavelength    dependence of the following approximate form; c×(wavelength in    microns)⁻³ where C<0.2 such that the contribution of this component    at 510 nm is generally less than 1.5 cm⁻¹.

FIG. 1 shows the absorption spectrum of a brown CVD diamond layer (curveB) and the components into which it can be decomposed. The first step insuch a spectral decomposition is the subtraction of the spectrum of atype Ib HPHT synthetic diamond (curve A), scaled so that the residualshows no 270 nm feature. The residual spectrum can then be decomposedinto a c×λ⁻³ component (curve C) and two overlapping bands of the kinddescribed above (curve D).

It has been found that the form of UV/visible spectra of CVD diamondgrown using a range of different processes can be well specified by sumsof the components described above, with different weighting factors forthe components in different cases. For the purposes of specifying theshape of the spectrum the contributions of the different components aregiven in the following ways.

270 nm: The peak 270 nm absorption coefficient of the type Ib componentis measured from a sloping baseline connecting the type Ib spectrumeither side of the 270 nm feature that extends over the approximaterange 235 nm-325 nm.

350 nm band: The peak absorption coefficient contribution of this band.

510 nm band: The peak absorption coefficient contribution of this band.

Ramp: The contribution of the c x λ⁻¹ component to the absorptioncoefficient at 510 nm

CIELAB Chromaticity Coordinate Derivation

The perceived colour of an object depends on thetransmittance/absorbance spectrum of the object, the spectral powerdistribution of the illumination source and the response curves of theobserver's eyes. The CIELAB chromaticity coordinates quoted in thispatent application have been derived in the way described below. Using astandard D65 illumination spectrum and standard (red, green and blue)response curves of the eye (G. Wyszecki and W.S. Stiles, John Wiley, NewYork-London-Sydney, 1967) CIE L*a*b* chromaticity coordinates of aparallel-sided plate of diamond have been derived from its transmittancespectrum using the relationships below, between 350 nm and 800 nm with adata interval of 1 nm:

-   S_(λ)=transmittance at wavelength λ-   L_(λ)=spectral power distribution of the illumination-   x_(λ)=red response function of the eye-   y_(λ)=green response function of the eye-   z_(λ)=blue response function of the eye    X=Σ _(λ) [S _(λ) x _(λ) L _(λ) ]/Y ₀    Y=Σ _(λ) [S _(λ) y _(λ) L _(λ) ]/Y ₀    Z=Σ _(λ) [S _(λ) z _(λ) L _(λ) ]/Y ₀    Where Y₀=Σ_(λ)y_(λ)L_(λ)    L*=116 (Y/Y ₀)^(1/3)−16=Lightness (for Y/Y₀>0,008856)    a*=500[(X/X ₀)^(1/3)−(Y/Y ₀)^(1/3)] (for X/X ₀>0.008856, Y/Y    ₀>0.008856)    b*=200[(Y/Y ₀)^(1/3)−(Z/Z ₀)^(1/3)] (for Z/Z ₀>0.008856)    C*=(a* ² +b* ²)^(1/2)=saturation    h _(ab) arctan (b*/a*)=hue angle

Modified versions of these equations must be used outside the limits ofY/Y₀, X/X₀ and Z/Z₀. The modified versions are given in a technicalreport prepared by the Commission Internationale de L'Eclairage(Colorimetry (1986)).

It is normal to plot a* and b* coordinates on a graph with a*corresponding to the x axis and b* corresponding to the y axis. Positivea* and b* values correspond respectively to red and yellow components tothe hue. Negative a* and b* values correspond respectively to green andblue components. The positive quadrant of the graph then covers huesranging from yellow through orange to red, with saturations (C*) givenby the distance from the origin.

It is possible to predict how the a*b* coordinates of diamond with agiven absorption coefficient spectrum will change as the optical pathlength is varied. In order to do this, the reflection loss must first besubtracted from the measured absorbance spectrum. The absorbance is thenscaled to allow for a different path length and then the reflection lossis added back on. The absorbance spectrum can then be converted to atransmittance spectrum which is used to derive the CIELAB coordinatesfor the new thickness. In this way the dependence of the hue, saturationand lightness on optical path length can be modelled to give anunderstanding of how the colour of diamond with given absorptionproperties per unit thickness will depend on the optical path length.

L*, the lightness forms the third dimension of the CIELAB colour space.It is important to understand the way in which the lightness andsaturation vary as the optical path length is changed for diamond withparticular optical absorption properties. This can be illustrated on acolour tone diagram in which L* is plotted along the y-axis and C* isplotted along the x-axis (such as FIG. 4). The method described in thepreceding paragraph can also be used to predict how the L*C* coordinatesof diamond with a given absorption coefficient spectrum depend on theoptical path length.

The C* (saturation) numbers can be divided into saturation ranges of 10C* units and assigned descriptive terms as below.  0-10 weak 10-20weak-moderate 20-30 moderate 30-40 moderate-strong 40-50 strong 50-60strong-very strong 60-70 very strong   70-80+ very very strong

Similarly the L* numbers can be divided up into lightness ranges asfollows:  5-15 very very dark 15-25 very dark 25-35 dark 35-45medium/dark 45-55 medium 55-65 light/medium 65-75 light 75-85 very light85-95 very very light

There are four basic colour tones defined by the following combinationsof lightness and saturation:

Bright: Light and high saturation, Pale: Light and low saturation,

Deep: High saturation and dark, Dull: Low saturation and dark,

FIG. 2 shows absorption spectra for four samples with orangish brown toorange-brown colour and grown to 1.7 mm thickness with differing growthconditions. These spectra have similar shapes but display a range ofdifferent absorption strengths, Thus, by altering the growth conditions,it is possible to tune the strength of absorption to achieve differentcolours for a given thickness of CVD layer. Similarly for a gemstoneproduced with a given size and cut, the colour can be tuned by alteringthe growth conditions.

The table below lists the strengths of the different contributions tothe four spectra shown in FIG. 2, defined in the way described earlier,together with the CIELAB information derived from the spectra. The hueangle, as given earlier, is defined as h_(ab)=arcitan (b*/a*).

Table Of Absorption Contributions And CIELAB Values Spectrum A B C D 270nm band (cm⁻¹) 0.93 1.3 1.6 1.6 350 nm band (cm⁻¹) 0.45 1.5 2.0 4.0 510nm band (cm⁻¹) 0.3 0.6 0.8 1.2 Ramp (cm⁻¹) 0.19 0.46 0.60 1.26 a* 1.21.7 2.7 4.0 b* 2.8 6.4 7.9 14.5 C* 3.0 6.7 8.3 15 L* 84 82 79 72 Hueangle (degrees) 68 75 71 75

FIGS. 3 and 4 show respectively an a*b* plot and an L*C* plot derived,in the way discussed above, from the absorption spectrum of one of the1.7 mm thick orangish brown CVD diamond layers (C). It can be seen thatthe L*C* curve runs between regions corresponding to pale, moderatelybright, deep and finally dull. Although this layer had a pale tone, theoptical properties of the diamond are such that thicker layers of suchdiamond, after skillfully polishing, can yield gemstones with a range ofdifferent possible tones and colours. This is illustrated by thepolished gemstones of examples 1, 3 and 4 that were given colour gradesof fancy light pink-brown, fancy dark orangish brown and fancy pinkbrown.

FIGS. 5 and 6 show CIELAB a*b* and L*C* plots for a range of samples ofsimilar thickness. They show that significant variations in hue,saturation and lightness result from differences in growth conditions.Thus the CVD process can be adjusted to control the colour that willresult for a polished stone of a given size and cut.

Collectors of natural fancy colour diamonds acknowledge that these aredesirable colours. In his book “Collecting and Classifying ColouredDiamonds” (Ashland Press, New York, 1998), Stephen Hofer describes theAurora Collection, one of the largest collections of natural fancycoloured diamonds. These diamonds are acknowledged to have desirablecolours and amongst them here are several with colours similar to thosewhich can be achieved in CVD synthetic diamond using the method of thisinvention. Some of these are listed below. In the two cases where theCIELAB data are given, the hue angles are very close to that for CVDsynthetic diamond of this invention.

Table Of Colour Descriptions Of Selected Diamonds From The AuroraCollection Hue angle Aurora no. Colour a* b* C* L* (degrees) 259 Lightpinkish orangish 5.1 11.1 12.2 70 65 (topaz) brown 231 Medium darkpinkish 8.3 18.3 20.1 43 66 (cinnamon) orangish brown 48 Medium pinkish(cinnamon) orangish brown 171 Dark orangish brown (cognac) 130 Very darkorangish (chestnut) brown 78 Medium-dark pinkish (cinnamon) orangishbrownLuminescence

Although the colour of a diamond is principally dependent on itsabsorption spectrum, it can also be influenced by its luminescenceproperties. This may be particularly the case for certain viewingconditions. For example, the luminescence will have the greatest effectwhen the diamond is viewed from a small distance under illumination withlight that contains a strong component in a wavelength range thatexcites the luminescence most efficiently.

The diamond of the present invention can show strong luminescence fromnitrogen-vacancy colour centres. The neutral and negatively charged N-Vcentres have their zero-phonon lines at 575 nm and 637 nm, respectively,and have absorption band systems on the shorter wavelength side of thesezero-phonon lines. Light of wavelengths within the range covered bythese absorption bands can be absorbed by these colour centres and giverise to luminescence with a spectrum which is characteristic of thesecentres. The luminescence from the neutral N-V centre is predominantlyorange. That from the negatively charged N-V centre is red.

The negatively charged N-V centre is a relatively strong absorber,giving rise to an absorption band system with a maximum at around 570nm. Some of the energy absorbed at these centres is re-emitted asluminescnce, In contrast, the neutral N-V centre has a very small effecton the absorption spectrum and the energy absorbed is typicallyconverted to luminescence with a high efficency.

N-V centres in the vicinity of an electron donor, such as singlesubstitutional nitrogen, are negatively charged, while isolated N-Vcentres are neutral. The effect of a given concentration of N-V centreson the colour of a diamond therefore depends on the concentration andrelative distribution of electron donors. For examnple, N-V centres indiamond containing a high concentration of N will contribute to thecolour predominantly via absorption of light by negatively charged N-Vcentres with a smaller contribution coming from luminescence. In thecase of diamond containing low concentrations of electron donors such asnitrogen, luminescence from neutral N-V centres can make a moreimportant contribution.

Luminescence Measurement And Quantification

As a result of variations in the importance of non-radiative paths,luminescence properties of diamond samples cannot in general be deduceddirectly from the concentrations of the various contributing centres asdetermined by absorption spectroscopy. Quantitative luminescenceproperties of diamond samples can, however be specified by normalisingthe integrated intensities of relevant luminescence lines or bandsrelative to the integrated intensity of diamond Raman scattering(nominally at 1332 cm⁻¹) collected under the same conditions.

The table below lists the results of quantitative luminescencemeasurements made on a range of single crystal CVD diamond samples ofthe invention. In each case, the measurements were made after removal ofthe {100} substrate on which they were grown. The growth conditionsfavoured the formation of predominantly <100> sector diamond sampleswith uniform luminescence properties as judged by luminescence imaging.Any small additional sectors with different luminescence properties wereremoved before the measurements were made.

The luminescence was excited at 77K with a 300 mW 514 nm argon ion laserbeam and spectra were recorded using a Spex 1404 spectrometer equippedwith a holographic grating (1800 grooves/mm) and a Hamamatsu R928photomultiplier. The data were corrected to allow for spectral responsefunction of the spectrometer system, derived using a standard lamp witha known spectral output. Normalised Normalised Sample I(575) I(637)I(637)/I(575) 404 1.929 6.880 3.566 407 5.808 17.65 3.039 409 3.11610.07 3.233 410 1.293 4.267 3.299 412 2.703 7.367 2.725 414 17.09 52.293.058 415 19.06 41.92 2.198 416 17.02 70.00 4.111 417 32.86 69.77 2.123418 29.34 61.31 2.089 423 6.985 7.019 1.004 424 51.41 101.8 1.981 42568.22 277.4 4.067 426 16.17 29.23 1.807 434 4.929 4.378 0.8883 4350.4982 1.223 2.455 437 0.3816 0.2224 0.5828 439 4.24 2.891 0.6818 5050.00954 0.04031 4.225 507c 0.3455 2.347 6.793 507b 0.106 0.03252 0.3068511b 4.611 4.211 0.9134 501 2.586 1.959 0.7577 512 7.282 7.686 1.055 5150.01886 0.01932 1.024 520 0.1802 0.5421 3.008 521 0.0402 0.03197 0.7936513 0.0243 0.01765 0.7240 509 25.22 13.87 0.5498 511c 0.0371 0.011120.2997 513b 1.091 1.262 1.155 513c 0.1717 0.2224 1.295 513d 1.992 0.76450.3836 510b 0.3922 0.6963 1.775 510c 0.1643 0.6268 3.815 510d 1.0910.6811 0.6238 514a 126.6 56.57 0.4466 514b 101.3 50.79 0.5012 514c 141.667.83 0.4789

It is important for the production of high crystalline quality (asherein defined) thick single crystal CVD diamond with propertiessuitable for coloured gem stones that growth takes place on a diamondsurface which is substantially free of crystal defects. In this context,defects primarily mean dislocations and micro cracks, but also includetwin boundaries, point defects not intrinsically associated with thedopant N atoms, low angle boundaries and any other extended disruptionto the crystal lattice. Preferably the substrate is a low birefringencetype Ia natural, Ib or IIa high pressure/high temperature syntheticdiamond or a CVD synthesised single crystal diamond.

The quality of growth on a substrate which is not substantially free ofdefects rapidly degrades as the layer grows thicker and as the defectstructures multiply, causing general crystal degradation, twinning andrenucleation.

The defect density is most easily characterised by optical evaluationafter using a plasma or chemical etch optimised to reveal the defects(referred to as a revealing plasma etch), using for example a briefplasma etch of the type described below. Two types of defects can berevealed:

-   -   1) Those intrinsic to the substrate material quality. In        selected natural diamond the density of these defects can be as        low as 50/mm² with more typical values being 10²/mm², whilst in        others it can be 10⁶/mm² or greater.    -   2) Those resulting from polishing, including dislocation        structures and microcracks forming chatter tracks along        polishing lines. The density of these can vary considerably over        a sample, with typical values ranging from about 10²/mm², up to        more than 10⁴/mm² in poorly polished regions or samples.

The preferred low density of defects is such that the density of surfaceetch features related to defects, as described above, is below5×10³/mm², and more preferably below 10²/mm².

The defect level at and below the substrate surface on which the CVDgrowth takes place may thus be minimised by careful preparation of thesubstrate, Included here under preparation is any process applied to thematerial from mine recovery (in the case of natural diamond) orsynthesis (in the case of synthetic material) as each stage caninfluence the defect density within the material at the plane which willultimately form the substrate surface when preparation as a substrate iscomplete. Particular processing steps may include conventional diamondprocesses such as mechanical sawing lapping and polishing (in thisapplication specifically optimised for low defect levels), and lessconventional techniques such as laser processing or ion implantation andlift off techniques, chemical/mechanical polishing and both liquid andplasma chemical processing techniques. In addition, the surface R_(Q)(root mean square deviation of surface profile from flat measured bystylus profilometer, preferably measured over 0,08 mm length) should beminimised, typical values prior to any plasma etch being no more than afew nanometers, i.e. less than 10 nanometers

One specific method of minimising the surface damage of the substrate,is to include an in situ plasma etch on the surface on which thehomoepitaxial diamond growth is to occur. In principle this etch neednot be in situ, nor immediately prior to the growth process, but thegreatest benefit is achieved if it is in situ, because it avoids anyrisk of further physical damage or chemical contamination. An in situetch is also generally most convenient when the growth process is alsoplasma based. The plasma etch can use similar conditions to thedeposition or diamond growing process, but with the absence of anycarbon containing source gas and generally at a slightly lowertemperature to give better control of the etch rate. For example, it canconsist of one or more of:

-   -   (i) an oxygen etch using predominantly hydrogen with optionally        a small amount of Ar and a required small amount of O₂. Typical        oxygen etch conditions are pressures of 50-450×10² Pa, an        etching gas containing an oxygen content of 1 to 4 percent, an        argon content of 0 to 30 percent and the balance hydrogen, all        percentages being by volume, with a substrate temperature        600-1100° C. (more typically 800° C.) and a typical duration of        3-60 minutes.    -   (ii) a hydrogen etch which is similar to (i) but where the        oxygen is absent.    -   (iii) alternative methods for the etch not solely based on        argon, hydrogen and oxygen may be used, for example, those        utilising halogens, other inert gases or nitrogen.

Typically the etch consists of an oxygen etch followed by a hydrogenetch and then moving directly into synthesis by the introduction of thecarbon source gas, The etch time/temperature is selected to enableremaining surface damage from processing to be removed, and for anysurface contaminants to be removed, but without forming a highlyroughened surface and without etching extensively along extended defectssuch as dislocations which intersect the surface and thus cause deeppits. As the etch is aggressive, it is particularly important for thisstage that the chamber design and material selection for its componentsbe such that no material is transferred by the plasma into the gas phaseor to the substrate surface. The hydrogen etch following the oxygen etchis less specific to crystal defects rounding off the angularities causedby the oxygen etch which aggressively attacks such defects and providinga smoother, better surface for subsequent growth.

The surface or surfaces of the diamond substrate on which the CVDdiamond growth occurs are preferably the {100}, {110}, {113} or {111}surfaces. Due to processing constraints, the actual sample surfaceorientation can differ from these ideal orientations up to 5°, and insome cases up to 10°, although this is less desirable as it adverselyaffects reproducibility.

It is also important in the method of the invention that the impuritycontent of the environment in which the CVO growth takes place isproperty controlled. More particularly, the diamond growth must takeplace in the presence of an atmosphere containing substantially nocontaminants other than the intentionally added nitrogen which should becontrolled to better than 500 parts per billion (as a molecular fractionof the total gas volume) or 5% in the gas phase, whichever is thelarger, and preferably to better than 300 parts per billion (as amolecular fraction of the total gas volume) or 3% in the gas phase,whichever is the larger, and more preferably to better than 100 partsper billion (as a molecular fraction of the total gas volume) or 1% inthe gas phase, whichever is the larger. Measurement of absolute andrelative nitrogen concentration in the gas phase at concentrations aslow as 100 ppb requires sophisticated monitoring equipment such as thatwhich can be achieved, for example, by gas chromatography. An example ofsuch a method is now described:

Standard gas chromatography (GC) art consists of: a gas sample stream isextracted from the point of interest using a narrow bore sample line,optimised for maximum flow velocity and minimum dead volume, and passedthrough the GC sample coil before being passed to waste. The GC samplecoil is a section of tube coiled up with a fixed and known volume(typically 1 cm³ for standard atmospheric pressure injection) which canbe switched from its location in the sample line into the carrier gas(high purity He) line feeding into the gas chromatography columns. Thisplaces a sample of gas of known volume into the gas flow entering thecolumn; in the art, this procedure is called sample injection.

The injected sample is carried by the carrier gas through the first GCcolumn (filled with a molecular sieve optimised for separation of simpleinorganic gases) and is partially separated but the high concentrationof primary gases (e.g. H₂, Ar) causes column saturation which makescomplete separation of, for example nitrogen difficult. The relevantsection of the effluent from the first column is then switched into thefeed of a second column, thereby avoiding the majority of the othergases being passed into the second column, avoiding column saturationand enabling complete separation of the target gas (N₂). This procedureis called “heart-cutting”.

The output flow of the second column is put through a dischargeionisation detector (DID), which detects the increase in leakage currentthrough the carrier gas caused by the presence of the sample. Chemicalstructure is identified by the gas residence time which is calibratedfrom standard gas mixtures. The response of the DID is linear over morethan 5 orders of magnitude, and is calibrated by use of specialcalibrated gas mixtures, typically in the range of 10-100 ppm, made bygravimetric analysis and then verified by the supplier. Linearity of theDID can be verified by careful dilution experiments.

This known art of gas chromatography has been further modified anddeveloped for this application as follows: The processes being analysedhere are typically operating at 50-500×10² Pa. Normal GC operation usesthe excess pressure over atmospheric pressure of the source gas to drivethe gas through the sample line. Here, the sample is driven by attachinga vacuum pump at the waste end of the line and the sample drawn throughat below atmospheric pressure. However, whilst the gas is flowing theline impedance can cause significant pressure drop in the line,affecting calibration and sensitivity. Consequently, between the samplecoil and the vacuum pump is placed a valve which is shut for a shortduration before sample injection in order to enable the pressure at thesample coil to stabilise and be measured by a pressure gauge. To ensurea sufficient mass of sample gas is injected, the sample coil volume isenlarged to about 5 cm³. Dependent on the design of the sample line,this technique can operate effectively down to pressures of about 70×10²Pa. Calibration of the GC is dependent on the mass of sample injected,and the greatest accuracy is obtained by calibrating the GC using thesame sample pressure as that available from the source under analysis.Very high standards of vacuum and gas handling practice must be observedto ensure that the measurements are correct.

The point of sampling may be upstream of the synthesis chamber tocharacterise the incoming gases, within the chamber to characterise thechamber environment, or downstream of the chamber.

The source gas may be any known in the art and will contain acarbon-containing material which dissociates producing radicals or otherreactive species. The gas mixture will also generally contain gasessuitable to provide hydrogen or a halogen in atomic form.

The dissociation of the source gas is preferably carried out usingmicrowave energy in a reactor examples of which are known in the art.However, the transfer of any impurities from the reactor should beminimised. A microwave system may be used to ensure that the plasma isplaced away from all surfaces except the substrate surface on whichdiamond growth is to occur and its mount (substrate carrier). Examplesof a preferred mount materials are: molybdenum, tungsten, silicon andsilicon carbide. Examples of preferred reactor chamber materials arestainless steel, aluminium, copper, gold and platinum.

A high plasma power density should be used resulting from high microwavepower (typically 3-60 kW, for substrate carrier diameters of 25-300 mm)and high gas pressures (50-500×10² Pa, and preferably 100-450×10² Pa).

Using the above conditions it has been possible to produce thick highquality single crystal CVD diamond layers with a desirable fancy colourusing nitrogen additions, calculated as molecular nitrogen, to the gasflow in the range 0.5 to 500 ppm. The range of nitrogen concentrationsfor which growth of fancy brown diamond is possible has a complexdependence on other parameters such as substrate temperatures pressureand gas composition.

Suitable conditions for synthesis of the material of the invention arebest illustrated by way of example.

EXAMPLE 1

Substrates suitable for synthesising single crystal CVD diamond of theinvention may be prepared as follows:

-   i) Selection of stock material (type Ia natural stones and type Ib    HPHT stones) was optimised on the basis of microscopic investigation    and birefringence imaging to identify substrates which were free of    strain and imperfections.-   ii) Laser sawing, lapping and polishing to minimise subsurface    defects using a method of a revealing plasma etch to determine the    defect levels being introduced by the processing.-   iii) After optimisation it was possible routinely to produce    substrates in which the density of defects measurable after a    revealing etch is dependent primarily on the material quality and is    below 5×10³/mm², and generally below 10²/mm². Substrates prepared by    this process are then used for the subsequent synthesis.

A high temperature/high pressure synthetic type 1b diamond was grown ina high pressure press, and as a substrate using the method describedabove to minimise substrate defects to form a polished plate withlateral dimensions 5 mm×5 mm and thickness 500 μm, with all faces {100}.The surface roughness R_(Q) at this stage was less than 1 nm. Thesubstrate was mounted on a tungsten substrate using a high temperaturediamond braze. This was introduced into a reactor and an etch and growthcycle commenced as described above, and more particularly:

-   1) The 2.45 GHz reactor was pre-fitted with point of use purifiers,    reducing unintentional contaminant species in the incoming gas    stream to below 80 ppb.-   2) An in situ oxygen plasma etch was performed using 15/75/600 sccm    (standard cubic centimetre per second) of O₂/Ar/H₂ at 263×10² Pa and    a substrate temperature of 730° C.-   3) This moved without interruption into a hydrogen etch with the    removal of the O₂ from the gas flow.-   4) This moved into the growth process by the addition of the carbon    source (in this case CH₄) and dopant gases. In this instance was CH₄    flowing at 42 sccm and 3 ppm N₂ (calculated as [N₂]/[All gases]    where [N₂] represents the number of moles of N₂ and [All gases]    represents the number of moles of all gases present) in the gas    phase. The substrate temperature was 830° C.-   5) On completion of the growth period, the substrate was removed    from the reactor and the CVD diamond layer removed from the    substrate.-   6) This layer identified as FN-1, was then polished to produce a    6×6×3 mm square cut synthetic diamond with weight 1.1 carats and    certified by a professional diamond grader to have a desirable fancy    light pink brown colour and a quality grade of VS1.-   7) FN-1 was further characterised by the data provided below:-   i) An optical absorption spectrum showing the characteristic broad    bands at 270 nm and approximately 355 nm and 510 nm. FIG. 7 shows    the decomposition of the original spectrum (curve A) into a type Ib    spectrum (curve B), a ramp component with a (wavelength)⁻³    dependence (curve C) and the two overlapping bands centred at 355    and 510 nm (curve D). The peak 270 nm absorption coeffident of the    type Ib component above a sloping baseline connecting the type Ib    spectrum either side of the 270 nm peak, is 0.67 cm⁻¹. The    (wavelengths)⁻³ component and the 510 nm band contribute 0.11 cm⁻¹    and 0.21 cm⁻¹ respectively at 510 nm. The 355 m band contributes    0.32 cm⁻¹ at its peak. FIGS. 8 and 9 show CIELAB hue and tone    diagrams respectively for diamond with the FN-1 absorption spectrum.    The CIELAB coordinates derived from the absorption spectrum of FN-1    were as follows: a*=1.8, b*=3.9, L*=81, C*=4.3 and hue angle =65    degrees.-   ii) Luminescence excited at 77 K with a 300 mW 514 nm Ar ion laser    showing the zero phonon lines at 575 and 637 nm with Raman    normalised intensities of 6.98 and 7.02 respectively.-   iii) The EPR spectra showing single substitutional nitrogen with    concentration 0.3 ppm.-   iv) X-ray rocking curves map, showing the angular spread of the    sample to be less than 20 arc sec.-   v) Raman spectrum showing a line width (FWHM) to be 2 cm⁻¹.-   vi) SIMS showed a total nitrogen concentration of 0.35 ppm

EXAMPLE 2

A 3.0 mm thick layer of CVD diamond was grown on a type Ib HPHTsynthetic diamond substrate prepared in the same way as described inexample 1 except with the following growth conditions:

-   -   (i) Etch temp of 718° C.    -   (ii) Growth conditions consisted of 32/25/600 sccm (standard        cubic centimetre per second) of CH₄/Ar/H₂ at 180×10² Pa and a        substrate temperature of 800° C. with 24 ppm added N₂.

After growth, the substrate was removed and the top and bottom surfacesof the were polished. UV/visible absorption spectra of the resulting CVDlayer, designated FN-2, were recorded and analysed into the componentsdiscussed in the detailed description of the invention. The results arelisted in the table below. Sample 270 nm 360 nm band 510 nm band RampFN-2 1.35 cm⁻¹ 1.05 cm⁻¹ 0.55 cm⁻¹ 0.31 cm⁻¹

The layer had a pale orangish brown colour and when the CIELABcoordinates were derived from the absorption spectrum, in the waydescribed in the detailed description of the invention, the followingresults were obtained. Hue angle Sample a* b* C* L* (degrees) FN-2 1.94.8 5.2 81 69

EXAMPLE 3

A 2.84 mm thick layer of CVD diamond was grown on a type Ib HPHTsynthetic diamond substrate prepared in the same way as described inexample 1 except with the following growth conditions:

-   -   (i) Etch temp of 710° C.    -   (ii) Growth conditions consisted of 42/25/600 sccm (standard        cubic centimetre per second) of CH₄/Ar/H₂ at 420×10² Pa and a        substrate temperature of 880° C. with 24 ppm added N₂.

The substrate was removed and resulting CVD layer, designated FN-3, waspolished into a rectangular cut CVD gemstone of 1.04 carats which wascertified by a professional diamond grader to have a desirable fancydark orangey brown colour and a quality grade of SI1.

The luminescence excited at 77 K with a 300 mW 514 nm Ar ion lasershowing the zero phonon lines at 575 and 637 nm with Raman normalisedintensities of 27.7 and 44.1 respectively.

EXAMPLE 4

A 3.53 mm thick layer of CVD diamond was grown on a type Ib HPHTsynthetic diamond substrate prepared in the same way as described inexample 1 except with the following growth conditions:

-   -   (i) Etch temp of 740° C.    -   (ii) Growth conditions consisted of 38/25/600 sccm (standard        cubic centimetre per second) of CH₄/Ar/H₂ at 283×10² Pa and a        substrate temperature of 860° C. with 21 ppm added N₂.

The substrate was removed and resulting CVD layer, designated FN-4, waspolished into a rectangular cut CVD gemstone of 1.04 carats which wascertified by a professional diamond grader to have a desirable fancypink brown colour and a quality grade of S13.

The luminescence excited at 77 K with a 300 mW 514 nm Ar ion lasershowing the zero phonon lines at 575 and 637 nm with Raman normalisedintensities of 15.26 and 21.03 respectively.

EXAMPLE 5

A 1.7 mm thick layer of CVD diamond was grown on a type Ib HPHTsynthetic diamond substrate prepared in the same way as described inexample 1 except with the following growth conditions:

-   -   (i) Etch temp of 716° C.    -   (ii) Growth conditions consisted of 160/40/3000 sccm (standard        cubic centimetre per second) of CH₄/Ar/H₂ at 260×10² Pa and a        substrate temperature of 823° C. with 3.8 ppm added N₂.

After growth, the substrate was removed and the top and bottom surfacesof the CVD diamond layer were polished. A UV/visible absorption spectrumof the resulting CVD layer, designated FN-5, was recorded (spectrum C inFIG. 2) and analysed into the components discussed in the detaileddescription of the invention. The results are listed in the table below.Sample 270 nm 360 nm band 510 nm band Ramp FN-5 1.60 cm⁻¹ 2.0 cm⁻¹ 0.80cm⁻¹ 0.60 cm⁻¹

The layer had a pale orangish brown colour and when the CIELABcoordinates were derived from the absorption spectrum, in the waydescribed in the detailed description of the invention, the followingresults were obtained. Hue angle Sample a* b* C* L* (degrees) FN-5 2.77.9 8.3 79 71

1. A diamond layer of single crystal CVD diamond which is coloured andwhich has a thickness greater than 1 mm, and in which at least one ofthe following is satisfied: (1) the hue angle is less than 80 degrees,(2) the absorption at 270 nm is from 0.1 cm⁻¹-30 cm⁻¹, and (3) theabsorption at 510 nm is from 0.1 cm⁻¹-10 cm⁻¹.
 2. A diamond layeraccording to claim 1 which has a fancy colour.
 3. A diamond layeraccording to claim 2 wherein the colour is a fancy colour with adominant brown component.
 4. A diamond layer according to claim 1wherein the colour is a fancy orangey brown, orange-brown, pinkishbrown, pink-brown or dark brown.
 5. A diamond layer according to claim 1wherein the hue angle is less than 80 degrees.
 6. A diamond layeraccording to claim 1 wherein the hue angle is less than 75 degrees.
 7. Adiamond layer according to claim 1 wherein the hue angle is less than 70degrees.
 8. A diamond layer according to claim 1 which has a thicknessgreater than 2 mm.
 9. A diamond layer according to claim 1 which has athickness greater than 3 mm.
 10. A layer of single crystal CVD diamondaccording to claim 1 which has one or more of the characteristics (i),(ii), (iii) observable in the majority volume of the layer, whichcomprises at least 55 percent of the whole volume of the layer: (i) Themajority volume of the layer contains one or more defect and impurityrelated colour centres that contribute to the absorption spectrum of thediamond as set out in the absorption coefficient column below:Absorption coefficient Designation Starts Ends Peak (at peak) 270 nm 220nm 325 nm 270 nm 0.1 cm⁻¹-30 cm⁻¹ 350 nm band 270 nm 450 nm 350 nm 0.3cm⁻¹-20 cm⁻¹ 510 nm band 420 nm 640 nm 510 nm 0.1 cm⁻¹-10 cm⁻¹ 570/637nm 500 nm 640 nm 570 nm 0.1 cm⁻¹-5 cm⁻¹  Designation Form of CurveAbsorption Coefficient Ramp Rising background of form Contribution atAbsorption coefficient 510 nm is: <3 cm⁻¹ (cm⁻¹) = C × λ⁻³ (C =constant, λin μm)

(ii) The majority volume of the layer contains defect and impurityrelated centres that contribute to the luminescence spectrum as set outin the Normalised luminescence intensity column of the table below, whenmeasured in the manner described herein using Ar ion 514 nm laserexcitation at 77 K: Normalised luminescence intensity of zero phononline Designation Starts Ends Peak at 77 K 575 nm 570 nm 680 nm 575 nm0.02-80  637 nm 635 nm 800 nm 637 nm 0.01-300

(iii) The majority volume of the CVD diamond layer exhibits a ratio ofnormalised 637 nm/575 nm luminescence, measured in the manner describedherein, which is in the range 0.2-10.
 11. A diamond layer according toclaim 10 wherein the majority volume comprises at least 80 percent ofthe whole volume of the layer.
 12. A diamond layer according to claim 10wherein the majority volume comprises at least 95 percent of the wholevolume of the layer.
 13. A diamond layer according to claim 10 whereinthe majority volume of the layer is formed from a single growth sector.14. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 270 nm has thecharacteristics: Absorption Designation Starts Ends Peak coefficient (atpeak) 270 nm 235 nm 325 nm 270 nm 0.4 cm⁻¹-10 cm⁻¹


15. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 270 nm has thecharacteristics: Absorption Designation Starts Ends Peak coefficient (atpeak) 270 nm 235 nm 325 nm 270 nm 0.8 cm⁻¹-6 cm⁻¹


16. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 350 nm has thecharacteristics: Absorption Designation Starts Ends Peak coefficient (atpeak) 350 nm band 270 nm 450 nm 350 nm 1.0 cm⁻¹-8 cm⁻¹


17. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 350 nm has thecharacteristics: Absorption Designation Starts Ends Peak coefficient (atpeak) 350 nm band 270 nm 450 nm 350 nm 1.5 cm⁻¹-6 cm⁻¹


18. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 510 nm has thecharacteristics: Absorption Designation Starts Ends Peak coefficient (atpeak) 510 nm band 420 nm 640 nm 510 nm 0.2 cm⁻¹-4 cm⁻¹


19. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 510 nm has thecharacteristics: Absorption Designation Starts Ends Peak coefficient (atpeak) 510 nm band 420 nm 640 nm 510 nm 0.4 cm⁻¹-2 cm⁻¹


20. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 570/637 nm hasthe characteristics: Absorption Designation Starts Ends Peak coefficient(at peak) 570/637 nm 500 nm 640 nm 570 nm 0.3 cm⁻¹-3 cm⁻¹


21. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the absorption spectrum of a diamond at 570/637 nm hasthe characteristics: Absorption Designation Starts Ends Peak coefficient(at peak) 570/637 nm 500 nm 640 nm 570 nm 0.3 cm⁻¹-1.5 cm⁻¹


22. A diamond layer according to claim 10 wherein the ramp has thecharacteristics: Absorption Designation Form of Curve coefficient (atpeak) Ramp Rising background of form Contribution at Absorptioncoefficient 510 nm is: <1.5 cm⁻¹ (cm⁻¹) = C × λ⁻³ (C = constant, λin μm)


23. A diamond layer according to claim 10 wherein the ramp has thecharacteristics: Absorption Designation Form of Curve coefficient (atpeak) Ramp Rising background of form Contribution at Absorptioncoefficient 510 nm is: <0.8 cm⁻¹ (cm⁻¹) = C × λ⁻³ (C = constant, λin μm)


24. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the luminescence spectrum of a diamond at 575 nm has thecharacteristics: Normalised luminescence intensity of zero phonon lineDesignation Starts Ends Peak at 77 K 575 nm 570 nm 680 nm 575 nm 0.05-60


25. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the luminescence spectrum of a diamond at 575 nm has thecharacteristics: Normalised luminescence intensity of zero phonon lineDesignation Starts Ends Peak at 77 K 575 nm 570 nm 680 nm 575 nm 0.2-40


26. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the luminescence spectrum of a diamond at 637 nm has thecharacteristics: Normalised luminescence intensity of zero phonon lineDesignation Starts Ends Peak at 77 K 637 nm 635 nm 800 nm 637 nm0.02-200


27. A diamond layer according to claim 10 wherein the colour centre thatcontributes to the luminescence spectrum of a diamond at 637 nm has thecharacteristics: Normalised luminescence intensity of zero phonon lineDesignation Starts Ends Peak at 77 K 637 nm 635 nm 800 nm 637 nm0.03-100


28. A diamond layer according to claim 10 wherein the ratio ofnormalised 637 nm/575 nm luminescence is in the range 0.5 to
 8. 29. Adiamond layer according to claim 10 wherein the ratio of normalised 637nm/575 nm luminescence is in the range 2 to
 5. 30. A layer of singlecrystal diamond which is coloured and which has, observable in themajority volume of the layer wherein the majority volume comprises atleast 55 percent of the whole volume of the layer, a low ramp as set outin the table below: Designation Form of Curve Absorption CoefficientRamp Rising background of form Contribution at Absorption coefficient510 nm is: <3 cm⁻¹ (cm⁻¹) = C × λ⁻³ (C = constant, λin μm)

and wherein the majority volume contains one or more of the defect andimpurity related colour centres that contribute to the absorptionspectrum of diamond as set out in the absorption coefficient column ofthe table below: Absorption coefficient Designation Starts Ends Peak (atpeak) 270 nm 220 nm 325 nm 270 nm 0.1 cm⁻¹-30 cm⁻¹ 350 nm band 270 nm450 nm 350 nm +/− 10 nm 0.3 cm⁻¹-20 cm⁻¹ 510 nm band 420 nm 640 nm 510nm +/− 50 nm 0.1 cm⁻¹-10 cm⁻¹ 570/637 nm 500 nm 640 nm 570 nm 0.1 cm⁻¹-5cm⁻¹


31. A method of producing a coloured single crystal diamond layerincludes the steps of providing a diamond substrate having a surfacewhich is substantially free of crystal defects, providing a source ofgas, dissociating the source gas to produce a synthesis atmosphere whichcontains 0,5 to 500 ppm nitrogen, calculated as molecular nitrogen, andallowing homoepitaxial diamond growth on the surface which issubstantially free of crystal defects.
 32. A method according to claim31 wherein the synthesis atmosphere contains 1 to 100 ppm nitrogen,calculated as molecular nitrogen.
 33. A method according to claim 31wherein the synthesis atmosphere contains nitrogen in an amount suitableto enhance the size of the {100} growth sector and reduce the size ofcompeting growth sectors.
 34. A method according to claim 31 wherein thedensity of defects is such of surface etch features related to defectsis below 5×10³/mm².
 35. A method according to claim 31 wherein thedensity of defects is such that the density of surface etch featuresrelated to defects is below 10²/mm².
 36. A method according to claim 31wherein the surface or surfaces of the diamond substrate on which CVDdiamond growth occurs is selected from the {100}, {110}, {113} and {111}surfaces.
 37. A diamond layer produced by a method according to claim31.
 38. A gemstone produced from a diamond layer according to claim 1 orclaim
 37. 39. A gemstone according to claim 38 with a quality grading ofSI1 or better.
 40. A gemstone according to claim 38 with a qualitygrading of VS1 or better.